1. Field of Invention
The present invention relates to a detection system for ion mobility spectrometer (IMS) in a technical field of detecting explosives, drugs and the like. In particular, the present invention relates to an electrode structure for a drift tube in IMS.
2. Description of Prior Art
The drift tube in the conventional IMS is generally made of a series of electrode plates and insulators. When ions move in the drift tube, part of the ions which move near the electrodes will hit the insulators and generate space charges. The accumulated charges will affect the uniform electric field in the drift tube and cause abnormality of the IMS. Such situation becomes even more noticeable when the electric field in the drift tube is weak or a large number of ions are collected together (e.g., the drift tube has gotten poisoned).
To avoid the occurrence of the above situation, the inner radius of an electrode is generally made smaller than that of an insulator during fabrication of an IMS. Unfortunately, this fabrication process reduces the effective usable size of the inner radius of the IMS, while its outline size becomes larger. So, limitation is imposed on the development of IMS products towards compactness, lightness and convenience.
In the past decades, the conventional IMS drift tube is mainly developed in consideration of how to form a uniform electric field (U.S. Pat. Nos. 6,229,143, 5,280,175), focusing (U.S. Pat. No. 7,164,122) and less cost (U.S. Pat. No. 6,051,832). No attempt is made to improve the drift tube with respect to the effect of charge accumulation. Only U.S. Pat. No. 4,390,784 describes a technique to eliminate the charge accumulation phenomenon in the drift tube.
U.S. 4,390,784 discloses that the accumulation of space charge in the drift tube can be abated by evaporation plating a resistance film in the glass tube, and it is also possible to reduce the outline size of the drift tube with its inner radius being the same as the conventional drift tube. This lays a foundation for the development of the IMS towards compactness.
In the drift tube using a resistance film, however, charges will hit onto the resistance film and cause a local potential change within the drift tube. This will increase the FWHM of detected spectrum and reduce the resolution of the IMS. Further, the method loses the ability to focus ions and requires a high production cost, when compared with the conventional drift tube using electrode rings.
FIG. 1 is a schematic diagram of a conventional drift tube using electrode rings. As shown in FIG. 1, to prevent ions from hitting onto the insulator and causing charge accumulation, the conventional drift tube has an inner radius of the electrode rings smaller than that of the insulator. As such, a channel is formed between two electrode rings, and most of the ions will hit onto the rings, instead of passing through the channel to hit the insulator. With such configuration, the outer radius of the drift tube becomes generally large.